The present invention relates to the conversion of a single-hull vessel into a double hull vessel. More specifically, the present invention relates to the conversion of a single hull oil tanker into a double hull ore carrying vessel. Yet more specifically, the present invention relates to the conversion of a single hull vessel into a double hull vessel, wherein the single hull vessel has pre-existing port and starboard wing tanks flanking a center cargo tank and an elevated floor is added in the bottom of the center cargo tank spanning the sidewalls of the wing tanks to provide a sealed compartment interposed between the center cargo tank and the bottom of the outer single hull. The present invention also includes the installation of a hatch in a deck of the vessel to provide access to the center cargo tank from above.
Currently, many single hull oil tanker marine vessels are approaching forced retirement because of the regulations in the shipping industry which require double hull vessels. At the present time, there are hundreds of single hull tankers which will soon no longer be able to trade in the United States or Europe where the present-day requirements call for double hull tankers. In fact, in many jurisdictions, there is a mandated phasing out of single hull tankers. There has also been a long period of under-investment in the tanker industry as a whole. Therefore, the value of these tankers has diminished. A significant number of these vessels are of high structural and operational quality. However, because of new legislation and other regulations, this large existing fleet of single hull vessels is being prematurely forced out of service.
The regulations requiring that oil tanker marine vessels have a double hull construction for the transport of oil and other hazardous liquids are currently being extended to cover other transport vessels, including those suited for carrying bulk materials such as iron ore. The number of double hull vessels suited for carrying these bulk materials is relatively small and new construction of such vessels is relatively expensive and a time consuming process. In fact, at the present time, there is a worldwide shortage of dry bulk shipping vessels including ore carriers, and a short-range forecast shows that limited plans for new construction portend a shortage for some time.
Thus, there is a need in the industry for double hull vessels configured to carry dry bulk materials while at the same time there is a significant inventory of existing single hull vessels which will be forced out of service.
There are examples in the prior art of efforts to convert single hull vessels to double hull vessels to meet these new regulations, as well as new designs for double hull vessels built from original designs therefor. One such example is shown in U.S. Pat. No. 3,399,645, issued in 1968, which discloses a double hull vessel originally built to design in recognition of the problems and risks associated with single hull vessels carrying environmentally unfriendly cargoes. In that patent, imperforated transverse bulkheads are provided to divide the cargo section into a row of center cargo tanks flanked by two rows of wing tanks. Then, an imperforated innerbottom is spaced from the bottom of the hull and extends the width and length of the cargo section to form the bottom of the center and wing tanks. Thus, with this construction, the center cargo tanks are isolated from the single outer hull by sealed compartments, and the bottom of the center cargo tanks are spaced above the bottom of the hull. However, this patent does not disclose or suggest a method for converting a single hull vessel to double hull.
Prior art U.S. patents that disclose apparatus and methods for converting single hull vessels to double hull vessels include U.S. Pat. Nos. 5,189,975 and 6,357,373. In the '975 patent, entitled “Method for reconfiguration tankers”, a method is disclosed for a mid-deck conversion which includes cutting the hull of a conventional single hull tanker longitudinally along a horizontal plane well below the normal laden water line and interposing a new mid-deck spacer member including a new transverse mid-deck between the lower and upper portions of the midship cargo section. Obviously, with this method, considerable effort and expense is required in order to literally split the original hull in two, separate the two pieces, and insert/install a new mid-deck construction. In the '373 patent issued in 2002, entitled “Rebuilt double hull vessel and method of rebuilding a single hull vessel into a double hull vessel”, an existing single hull vessel is rebuilt by cutting and removing the topside decking and internal vessel structure and inserting an inner hull structure within a volume defined by the original hull. The new inner hull is prefabricated as a plurality of modular sections, and the prefabricated modules are fitted over the top of the existing bottom framing members and joined to the existing framing members at the sides. This is also described as creating a new trunk structure which is “dropped” into the opening formed in the topside deck plating when the cut out section of deck plating is removed. Again, as with the '975 patent, substantial re-configuration of the original structure is required, involving significant effort and expense.
Still another prior art publication is a Japanese abstract, publication number 61-024685 published in 1986 and entitled “Method of reconstructing existing tanker into double hull tanker”. Reading from an abbreviated translation, this reference teaches a method requiring the cutting off of the side bottom portion of the hull and adding an inner bottom and a longitudinal bulkhead. Again, substantial re-configuring of the original vessel is required including cutting through the original hull.
The disclosures of these prior art references are incorporated herein by reference, but are not believed to detract from the patentability of the present invention.